It depends what sort of diet pills you're looking for. Over-the-counter crap will most likely be ineffective. If you're looking for a prescription from a doctor, you might (and I use the word 'might' lightly, for it only happens in rare cases) be able to get a drug called "Adderall". Adderall is a methamphetamine - yes, it's 'speed', and it comes with all the side effects of doing speed. The drug is often used to treat patients with attention defecit/hyperactivity disorder (ADHD). If you take it, you will lose weight. You will also, in all likelihood, become addicted to it. Patients who suddenly stop taking Adderall face severe withdrawal symptoms, including seizures. Look it up on Wikipedia.
Watch the movie 'Requiem for a Dream' if you want to see how one woman's quest to lose weight using pills works out.
Long story short, I don't think there's any safe 'diet pill' out there, although I'm sure those money-hungry pharmaceutical companies are getting close to developing one.